Hey, having some issues with a system 11 CPU board from a High Speed. Specifically random switch closures. Lead me to investigate the switch matrix.
I found that two of the eight matrix drive lines(1J8-4 and 1J8-7) are pulsing at twice the frequency as the rest.
I suspect the PIA at U38, so I replace it. No change.
I suspect something up with U40 or the transistor drive stuff, so I bend up the 8 legs of the PIA that feed U40 (pb0-pb7) to not make contact with the socket and retest at pins 13 and 15 on U38 (pb3-pb5). No change. (this has eliminated every thing after the PIA)
I’m at a loss now, so I try a new set of game roms at U26 and U27, no change.
Now I really don’t know where to go. Any Ideas what could cause two of the lines to cycle at twice the frequency? Something corrupting the data or address bus?
Any thoughts would be appreciated, I’m pulling my hair out on this one.
Thanks for now
I found that two of the eight matrix drive lines(1J8-4 and 1J8-7) are pulsing at twice the frequency as the rest.
I suspect the PIA at U38, so I replace it. No change.
I suspect something up with U40 or the transistor drive stuff, so I bend up the 8 legs of the PIA that feed U40 (pb0-pb7) to not make contact with the socket and retest at pins 13 and 15 on U38 (pb3-pb5). No change. (this has eliminated every thing after the PIA)
I’m at a loss now, so I try a new set of game roms at U26 and U27, no change.
Now I really don’t know where to go. Any Ideas what could cause two of the lines to cycle at twice the frequency? Something corrupting the data or address bus?
Any thoughts would be appreciated, I’m pulling my hair out on this one.
Thanks for now
